1. Technical Field
This invention relates to hollow airfoils in general, and to geometries of trailing edge cooling holes within hollow airfoils in particular.
2. Background Information
In modern axial gas turbine engines, turbine rotor blades and stator vanes require extensive cooling. A typical rotor blade or stator vane airfoil includes a serpentine arrangement of passages connected to a cooling air source, such as the compressor. Air bled from a compressor stage provides a favorable cooling medium because its pressure is higher and temperature lower than the core gas traveling through the turbine; the higher pressure forces the compressor air through the passages within the component and the lower temperature transfers heat away from the component. Cooling air ultimately exits the airfoil via cooling holes in the airfoil walls or cooling ports distributed along the trailing edge. Cooling is particularly critical along the trailing edge, where the airfoil narrows considerably. Most airfoil designs include a line of closely packed cooling ports in the exterior surface of the pressure side wall, distributed along the entire span of the airfoil. A relatively small pressure drop across each of the closely packed ports encourages the formation of a boundary layer of cooling air (film cooling) aft of the ports that helps cool and protect the aerodynamically desirable narrow trailing edge.
In addition to cooling, turbine rotor blade and stator vane airfoils must also accommodate high cycle fatigue (HCF) resulting from vibratory loadings. This is particularly true along the narrow trailing edge, where each of the closely packed cooling ports represents a significant stress concentration. Left unchecked, HCF can create stress fractures which can eventually compromise the mechanical integrity of the airfoil. FIG. 1 shows a sectional view of a conventional trailing edge with a cooling port in the pressure side wall, connected to an internal cavity via a passage. The width of the pressure side wall narrows considerably adjacent the cooling port, making that portion of the pressure side wall particularly susceptible to HCF. Moving the port forward to increase the wall thickness minimizes susceptibility to HCF, but also adversely affects film cooling aft of the port (film cooling effectiveness generally degrades with distance).
Hence, what is needed is an airfoil with trailing edge cooling apparatus that inhibits HCF, one that enhances downstream film cooling, and one that can be readily manufactured.